1. Field of the Invention
The present invention relates a duplex pipe end working method and a duplex pipe end working apparatus in which an end portion of a duplex pipe comprising an outer pipe, an inner pipe and connecting ribs molded into an integral body by extrusion working or by drawing working is skinned, more specifically, the outer pipe and connecting ribs of the duplex pipe are respectively cut in such a manner that the inner pipe can be projected outwardly of the outer pipe and can be thereby exposed to the outside.
2. Description of the Related Art
As conventionally known, a duplex pipe comprises an outer pipe through which a first fluid flows and an inner pipe disposed inside the outer pipe, through which a second fluid flows. According to the manufacturing method thereof, the duplex pipe can be roughly classified into two types. That is, one of them is a duplex pipe of a type that an outer pipe and an inner pipe are manufactured individually, and the outer pipe is drawing worked with the inner pipe inserted into the outer pipe, whereby projections provided on the inner wall of the outer pipe are pressure contacted with the outer wall of the inner pipe; and, the other is a duplex pipe of a type that an outer pipe, an inner pipe and connecting ribs for connecting together the inner and outer pipes are molded into an integral body by extrusion working or by drawing working. In recent years, from the viewpoint of reduction in the manufacturing cost thereof, there has been widely used the duplex pipe of the latter type that the outer pipe, inner pipe and connecting ribs are molded into an integral body.
In the case of the duplex pipe, a joint of a flange type or a nut-union type is connected to the end portion thereof before it is actually used. Thus, in the case of the duplex pipe of the latter type, for connection of such joint, a duplex pipe end working operation is performed on the end portion of the duplex pipe, that is, the end portions of the outer pipe and connecting ribs are respectively cut in such a manner that the inner pipe can be projected outwardly of the outer pipe.
Now, FIGS. 10A to 10C are respectively views of a conventional duplex pipe, showing how the end portion of the duplex pipe is worked. As shown in FIG. 10A, a duplex pipe 10, which is held by chucking an outer pipe 11, is rotated and, while moving a cutting tool 55 in the axial direction of the duplex pipe 10 (in FIG. 10A, in the direction of an outline arrow mark) from the end face of the duplex pipe 10, the outer pipe 11 and connecting ribs 13 are cut. In case where the cutting tool 55 is moved up to a position corresponding to the exposed length of the inner pipe 12, as shown in FIG. 10B, the outer pipe 11 and connecting ribs 13 are cut so that the inner pipe 12 can be projected outwardly of the outer pipe 11 and can be thereby exposed to the outside.
By the way, in the case of the duplex pipe 10 in which the outer pipe 11, inner pipe 12 and connecting ribs 13 are molded into an integral body, due to its manufacturing method using extrusion working or drawing working, it is difficult that the center line of the outer pipe 11 is made to coincide with the center line of the inner pipe 12.
As in the above-mentioned conventional duplex pipe end working method, in case where, while chucking the outer pipe 11 and rotating the duplex pipe 10, the outer pipe 11 and connecting ribs 13 are cut according to an ordinary pipe working method using the cutting tool 55, as shown in FIG. 10C, there are produced an excessively worked portion a and an unworked portion b and, therefore, it is difficult to work the inner pipe 12 in such a manner that the thickness thereof can be made uniform over the whole periphery thereof. The reason for this is that, in case where the duplex pipe 10 is worked with the outer pipe 11 chucked, the outer and inner pipes 11 and 12 are not positioned on the same concentric axis; and, therefore, even in case where the cutting tool 55 is moved with the outer pipe 11 as a reference, the positional relation of the inner pipe 12 and cutting tool 55 cannot be made constant. Also, since an amount or a direction in which the center line of the inner pipe 12 is displaced from the center line of the outer pipe 11 is not constant, it is impossible to bring the inner pipe 12 to an on-center position while chucking the outer pipe 11.
In case where the thickness of the exposed inner pipe 12 is not uniform, when the inner pipe 12 is punched or rolled, a crack occurs in the inner pipe 12. For this reason, according to the conventional working method, in fact, it is impossible to execute a punching operation on the exposed inner pipe end portion after the duplex pipe end portion is worked.
Further, as enlargedly shown in FIG. 10C, burrs 56 occur in the end portions of the connecting ribs 13. While the burrs 56 remain unremoved, the flow resistance of the fluid flowing through the outer pipe 11 is increased and thus the flowing noise of the fluid when it flows through the outer pipe 11 is also increased. Therefore, after completion of the cutting operation by the cutting tool 55, it is necessary to execute a post-treatment operation for removing the burrs 56. Also, because there are present many portions from which the burrs 56 must be removed, it is difficult to control the cleaning of the cut powders. In case where the cleaning of the cut powders is insufficient, the connecting ribs 13 can be contaminated, which results in the poor sealing.
Moreover, since the outer pipe 11 and connecting ribs 13 are respectively cut away in their end portions by the amounts that correspond to the exposed portion of the inner pipe 12, the cutting margins of the outer pipe 11 and connecting ribs 13 are large, which gives rise to production of a large amount of cut powders. Due to this, the above-mentioned cleaning control of the cut powders is made more troublesome and difficult.